I don't carry around a water bottle, thermos, or umbrella. So what to do with the rear straps on the Brain Bag? The straps barely take a 4" diameter bag, so at 5" diameter, the tripod quiver is too big for me (and the shortest is too long to nestle well in the space in the back of the pack). What to do? The answer is obvious of course: have my own bag made. :-)
Gray ripstop nylon, orange cord, with a strap in the bottom and a vertically-placed D-ring in the back, both of them to affix it permanently to the Brain Bag if so desired. Weighs nothing, folds into a space half the size of a cigarette pack. Stuffed to the gills with baby socks as shown below, it's about 3.75" in diameter by 12 inches high. Easily wide enough for me to put my (large) hand down. so I can use it for coke bottles etc. too. What do you think?
In the last picture, it's holding a wine bottle.